A planar structure of a pixel unit of an array substrate is illustrated in FIG. 1, and FIG. 1 shows two adjacent pixel units. Each pixel unit comprises a pixel electrode 1, a gate electrode 2, a gate electrode line 7 connected with the gate electrode 2, a source electrode 3, a data line 6 connected with the source electrode 3, and a drain electrode 4 connected with the pixel electrode 1. After powered on, the data line 7 charges the pixel electrode 1 via the drain electrode 4 of a thin film transistor, liquid crystal molecules 5 undergo deflection under the action of an electric field; at this time, a sectional view of the edge of the pixel unit is illustrated in FIGS. 2 and 3. FIGS. 2 and 3 only illustrate a black matrix on an opposed substrate (for example, a color filter substrate) opposite to the array substrate. In order to increase the transmission ratio of a liquid crystal display panel, the width of the black matrix 11 on the color filter substrate can be reduced. Combined with FIGS. 1 to 3, FIG. 2 is a sectional view of the left side edge of the pixel unit along A-A′ illustrated in FIG. 1, and FIG. 3 is a sectional view of the right side edge of the pixel unit along A-A′ illustrated in FIG. 1. As illustrated in FIGS. 2 and 3, if the width of the black matrix 11 is reduced, because the fringing electric field lines at the edge of the pixel unit are not uniform and generate bend, the orientations of liquid crystal molecules distributed at the edge of the pixel unit are not in accordance with the orientations of the liquid crystal molecules of the middle portion of the pixel unit, and there occurs irregular arrangement, which leads to light leakage at the edge of the pixel unit and affects the display effect. Therefore, in order to prevent the light leakage, as illustrated in FIGS. 2 and 3, the edge of the black matrix 11 slightly covers the edge of the pixel electrode 1 in a vertical direction, i.e., the two are partially overlapped in the vertical direction.
Here, the light leakage phenomenon occurs at a long edge side of the pixel unit, i.e., the left or right side of the pixel unit illustrated in FIG. 1; it is because the liquid crystal molecules 5 are arranged along the long edges of the pixel unit; when viewed from the left and right directions of the pixel unit, the liquid crystal molecules 5 have smaller birefringence in the directions, so it is easy for leak light to occur. However, when viewed from the up and down directions of the pixel unit (as illustrated in FIG. 1), the liquid crystal molecules have bigger birefringence in the directions, so it is not easy for leak light to occur.